This invention relates to a winch system, and more particularly to a winch system for use in a deep shaft.
FIG. 1 is a diagram showing a conventional winch system. A d.c. motor 10 drives a drum 12 that raises and lowers heavy loads packed in skips 16 and 18 attached to rope 14. A thyristor Leonardo device 20 controls d.c. motor 10, and a current detecting device 26 detects the armature current of d.c. motor 10. A current setting circuit 28 sets the electric current for motor 10 (connections not shown), and a current comparison circuit 30 compares the armature current detected by current detecting device 26 with the current set by current setting circuit 28. If the values of these two are equal, current comparison circuit 30 outputs a brake release command to brake release circuit 22 which controls the release of brake 24 on clamping drum 12.
In FIG. 1, a load is packed in skip 16, while skip 18 is unloaded. Then the loaded skip 16 is wound up, and a high output is required of d.c. motor 10. However, since the weight of the load of skip 16 is not known at the time that d.c. motor 10 starts up, the torque required to raise skip 16 is unknown. In the case of a machine moving a load horizontally, there is no particular problem if the motor only starts after the brake is released. However, in the case of a vertical winch system, if the winding operation is started only after brake 24 is first released, the armature current of d.c. motor 1 cannot generate a sufficient torque and skip 16 will begin to fall. The downward movement of skip 16 stops when the torque developed by motor 10 equals the downward torque exerted by the weight of skip 16 on drum 12 Subsequently, skip 16 only starts to be wound upwards when a net upwards torque is generated. This phenomenon is very dangerous; therefore, before releasing brake 24, a torque is generated by causing a constant armature current of a certain magnitude (for example, 200 percent of the rated magnitude) to flow in d.c. motor 10. In this way, the above-described phenomenon, called "fall-back," can be prevented from occuring.
Even though the weight of the load packed in skip 16 is not necessarily always the same, in the conventional system the torque that is generated by d.c. motor 10 prior to releasing brake 24 is always the same. This can result in several dangerous situations. If the load is heavier than normal, the starting torque will be insufficient, giving rise to the risk that the above described "fall-back" phenomenon will occur. On the other hand, if the load is lighter than normal, there will be an excess torque applied and the skip will start rapidly with a jerk. With the conventional system there is the problem that there will not be a smooth starting characteristic.
If the weight of the load, or total weight of skip 16 including the load, could be determined, there would be no problem In elevators etc., the weight of the cage is monitored using a load cell or the like. However, in the case of a winch system, it is much more difficult to adopt this technique than it is in the case of an elevator or the like. Specifically, in the case of a mining shaft, the vertical distance is long, often reaching about 2,000 m, so even if a weight detector could be fitted to the skip itself, the method of feeding electricity to it and handling the signal line would be a problem. Even if this problem could be solved, it would be necessary to allow for an electric cable of 2,000 m which must move up and down with the skip. This would be unsatisfactory because of the increase in the capacity of the shaft winding system which would be necessary.